linux-kernel - Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the fast path

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Date:	Sun, 17 Apr 2016 23:19:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ben Hutchings <ben@...adent.org.uk>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the
 fast path

On 04/17/16 23:14, Andy Lutomirski wrote:
>>
>> It's not "weird", it is the ABI as defined.  We have to do this for all
>> the system call arguments, too; you just don't notice it because the
>> compiler does it for us.  Some other architectures, e.g. s390, has the
>> opposite convention where the caller is responsible for normalizing the
>> result; in that case we have to do it *again* in the kernel, which is
>> one of the major reasons for the SYSCALL_*() macros.
> 
> What ABI?
> 

The C ABI for int.  I hadn't seen the below, because I think syscall(3)
is just braindamaged, but the odds are that if we'd ever use the upper
32 bits for anything we'd be in a world of hurt, so that would be highly
theoretical IMO.  Bit 31 might be possible, but I wouldn't really want
to brave it unless we really have no choice.

> Also, the behavior in which fail the syscall if any high bits are set
> is faster -- it's one fewer instruction.  Admittedly, the CPU can
> probably do that instruction for free, but still...

Yes, it can; at least on any remotely modern hardware.

	-hpa